Have you ever felt like you’re waiting in line at a coffee shop for JavaScript to fetch your latte? Asynchronous programming can often feel like that—multiple orders being processed at the same time can leave you stuck waiting. Fortunately, tools like Promises and async/await ensure the process stays smooth and efficient, letting your code keep moving without delays.
In this guide, we’ll break down how Promises work, why async/await was introduced, and how it simplifies writing asynchronous code. Whether you’re a beginner trying to grasp these concepts or looking for clarity on when to use each approach, this article will help you master the basics.
What Are Promises?
Promises are a foundational concept in JavaScript for handling asynchronous operations. At their core, a Promise represents a value that might be available now, later, or never. Think of it like a tracking number for a package: while you don’t have the package yet, the tracking number gives you confidence that it’s on its way (or lets you know if something went wrong).
Building upon the "now, later, or never" narrative, a Promise actually operates in one of three states:
- Pending: The asynchronous operation hasn’t been completed yet.
- Fulfilled: The operation was completed successfully, and the Promise now holds the result.
- Rejected: Something went wrong, and the Promise provides an error.
Creating and working with Promises involves a simple API. Here’s how you can define a Promise:
const fetchData = new Promise((resolve, reject) => {
setTimeout(() => {
const data = { id: 1, name: "JavaScript Basics" };
resolve(data); // Simulates a successful operation
// reject("Error: Unable to fetch data"); // Simulates a failure
}, 1000);
});
To handle the result, you can chain .then(), .catch(), and .finally() methods to the Promise object:
fetchData
.then((data) => {
console.log("Data received:", data);
})
.catch((error) => {
console.error(error);
})
.finally(() => {
console.log("Operation complete.");
});
The callback in the then() method is executed when the Promise resolves with a successful result. The callback in the .catch() method is executed when the Promise resolves with a failed result, and the callback in the finally() method is executed after the Promise has resolved, irrespective of the result of the resolution.
The Benefits of Promises
Promises provide a cleaner alternative to deeply nested callbacks, often referred to as “callback hell.” Instead of stacking callbacks, Promises allow chaining, making the flow of operations easier to follow:
doTask1()
.then((result1) => doTask2(result1))
.then((result2) => doTask3(result2))
.catch((error) => console.error("An error occurred:", error));
Here's what this same code would have looked like if it had been written using traditional callbacks:
doTask1((error1, result1) => {
if (error1) {
console.error("An error occurred:", error1);
return;
}
doTask2(result1, (error2, result2) => {
if (error2) {
console.error("An error occurred:", error2);
return;
}
doTask3(result2, (error3, result3) => {
if (error3) {
console.error("An error occurred:", error3);
return;
}
console.log("Final result:", result3);
});
});
});
Confusing, isn't it? This is why Promises were a game-changer in JavaScript coding standards when they were introduced.
The Shortcomings of Promises
While Promises greatly improved upon traditional callback functions, they did not come without their own unique challenges. Despite their benefits, they can become unwieldy in complex scenarios, resulting in verbose code and debugging difficulties.
Even with .then() chaining, Promises can result in cluttered code when dealing with multiple asynchronous operations. For example, managing sequential operations with .then() blocks and error handling using .catch() can feel repetitive and harder to follow.
doTask1()
.then((result1) => doTask2(result1))
.catch(Task1Error, (error) => console.error("An error occurred in task 1:", error));
.then((result2) => doTask3(result2))
.catch(Task2Error, (error) => console.error("An error occurred in task 2:", error));
.catch((error) => console.error("An error occurred in the chain:", error));
While cleaner than nested callbacks, the chaining syntax is still verbose, especially when detailed custom error-handling logic is required. Moreover, forgetting to add a .catch() at the end of a chain can lead to silent failures, making debugging tricky.
Furthermore, stack traces in Promises are not as intuitive as those in synchronous code. When an error occurs, the stack trace may not clearly indicate where the issue originated in your asynchronous flow.
Lastly, although Promises help reduce callback hell, they can still result in complexity when tasks are interdependent. Nested .then() blocks can creep back in for certain use cases, bringing back some of the readability challenges they were meant to solve.
Enter async/await
Asynchronous programming in JavaScript took a giant leap forward with the introduction of async/await in ES2017 (ES8). Built on top of Promises, async/await allows developers to write asynchronous code that looks and behaves more like synchronous code. This makes it a real game-changer for improving readability, simplifying error handling, and reducing verbosity.
What is async/await?
Async/await is a syntax designed to make asynchronous code easier to understand and maintain.
The async keyword is used to declare a function that always returns a Promise. Within this function, the await keyword pauses execution until a Promise is resolved or rejected. This results in a flow that feels linear and intuitive, even for complex asynchronous operations.
Here’s an example of how async/await simplifies the same code example you saw above:
async function executeTasks() {
try {
const result1 = await doTask1();
const result2 = await doTask2(result1);
const result3 = await doTask3(result2);
console.log("All tasks completed successfully:", result3);
} catch (error) {
if (error instanceof Task1Error) {
console.error("An error occurred in task 1:", error);
} else if (error instanceof Task2Error) {
console.error("An error occurred in task 2:", error);
} else {
console.error("An error occurred in the chain:", error);
}
}
}
executeTasks();
Async/await eliminates the need for .then() chains, allowing code to flow sequentially. This makes it easier to follow the logic, especially for tasks that need to be executed one after another.
With Promises, errors must be caught at every level of the chain using .catch(). Async/await, on the other hand, consolidates error handling using try/catch, reducing repetition and improving clarity.
Async/await produces more intuitive stack traces than Promises. When an error occurs, the trace reflects the actual function call hierarchy, making debugging less frustrating. On the whole, async/await feels more "natural" because it aligns with how synchronous code is written.
Comparing Promises and async/await
As you've already seen, Async/await shines when it comes to readability, especially for sequential operations. Promises, with their .then() and .catch() chaining, can quickly become verbose or complex. In contrast, async/await code is easier to follow as it mimics a synchronous structure.
Flexibility
Promises still have their place, particularly for concurrent tasks. Methods like Promise.all() and Promise.race() are more efficient for running multiple asynchronous operations in parallel. Async/await can handle such cases too, but it requires extra logic to achieve the same result.
// A Promise.all() example
Promise.all([task1(), task2(), task3()])
.then((results) => console.log("All tasks completed:", results))
.catch((error) => console.error("An error occurred:", error));
Error handling
While centralized error handling with a single .catch() works well for linear chains of Promises, it is recommended to use distributed .catch calls for different error types across chains for best readability.
On the other hand, a try/catch block offers a more natural structure for handling errors, especially when dealing with sequential tasks.
Performance
In terms of performance, async/await is essentially equivalent to Promises since it is built on top of them. However, for tasks requiring concurrency, Promise.all() can be more efficient because it allows multiple Promises to execute in parallel, failing fast if any Promise rejects.
When to Use Which
If your tasks involve a lot of concurrent operations, such as fetching data from multiple APIs simultaneously, Promises are most probably the better choice. If your asynchronous code does not involve a lot of chaining, Promises would be well-suited in that situation as well because of its simplicity.
On the other hand, async/await excels in situations where a lot of tasks need to be executed sequentially or when readability and maintainability are priorities. For example, if you have a series of dependent operations, such as fetching data, transforming it, and saving it, async/await offers a clean and synchronous structure. This makes it easier to follow the flow of operations and simplifies centralized error handling with try/catch blocks. Async/await is especially useful for beginners or teams prioritizing readable code.
Conclusion
JavaScript offers two powerful tools for managing asynchronous operations: Promises and async/await. Promises revolutionized the way developers handle asynchronous tasks, resolving issues like callback hell and enabling chaining. Async/await builds on Promises, providing a cleaner syntax that feels more natural and intuitive, especially for sequential tasks.
Now that you’ve explored both approaches, you’re equipped to choose the best one for your needs. Try converting a Promise-based function to async/await and observe the difference in readability!
For more information, check out the MDN Promise documentation or experiment with an interactive coding sandbox!
Top comments (1)
You can also have a unique
catchafter multiplethenlike that: